The invention also relates to a device comprising an electric motor and force-determining means for determining a drive force produced by an electric motor during operation.
Finally, the invention also relates to a mechanism for limiting the closing force of a component of a motor vehicle, which is driven with the aid of an electric motor, comprising such a device.
Determination of a drive force produced by a rotating electric motor can often be of interest and is particularly significant in motor vehicles if the movement of a component of a motor vehicle, such as a window or a sliding roof in particular, but also a vehicle seat, etc., is to be detected. One particular application in this connection is monitoring in terms of trapping of body parts, and generally objects, by the moved component, a corresponding reaction to limit or switch off the drive force (closing force) then being provided in order to avoid damage. Optimally accurate detection of the drive force is required for a closing force limitation of this kind, for example in the case of sliding roofs or window openers.
To limit the closing force, the procedure is conventionally such that measured values based on the speed of the drive movement (or preferably the rotational speed of the electric motor) are provided with the aid of Hall sensors in order to draw conclusions about the exerted forces from these, cf. for example FR 2663798 A. Furthermore, it has also already been proposed to measure the motor current and define a threshold value for determining a trapped state. This proposal is more advantageous insofar as no Hall sensors are required, so the apparatus-related complexity and costs are reduced. The estimated correlative force value FAct should be proportional to the current in this case, for instance according to the following equation:
                              F          Act                =                                            u              ¨                        r                    *                      k            m                    *                      I            a                                              (        1        )            
The term
            u      ¨        r    *      k    m  herein can be replaced by a correction or proportionality factor kx, so equation (1) can also be written as follows:FAct=kx*Ia 
In the above equation (1) FAct designates the drive force to be determined, for example on the cable pull of the window opener, ü designates the gear ratio, r the radius of the cable drum of the drive system for the window opener and km a motor torque constant. Ia moreover designates the motor current.
One problem in this connection is that it is not possible to more accurately estimate the drive force in terms of a possible trapping incident as the power supply source, in the case of motor vehicles the motor vehicle battery in other words, is frequently affected by voltage fluctuations. Voltage fluctuations of this kind, for example in the form of sudden voltage jumps, naturally have an effect on the motor current and therewith on detection of the drive force and prevent smooth operation, for example in the case of limiting the closing force on a motor vehicle component.